Total Knee Arthroplasty - part 5 ppt

156 III Kinematics 23 FE axis ⊡ Fig 23-5 The average patella shift in the frontal plane along the FE axis is 4.3 mm Open circles represent the patellar position at flexion angles between 0° and 15°; solid circles represent the patellar position at flexion angles between 20° and 90° (From [5]) ⊡ Fig 23-4 Schematic of a distal femur (dark line) and the path of the patellar tracking from full extension to 90° flexion (right) The center of the arc describing the patellar path is located at a distance (approximately 10 mm anteriorly and 10 mm proximally) from the FE axis [3] R Radius of the epicondylar circle, r radius of the arc describing the patellar tracking clinical observation that external rotation of the femoral component aids patellar tracking in TKA Patellar Component Forces Patellar tracking and patellar component failure are important aspects of total knee arthroplasty While fluoroscopic in vivo studies provide data regarding tibiofemoral kinematics, they not show the effect of tibiofemoral kinematics on the patellofemoral articulation In vitro studies can provide helpful information on patellar kinematics and patellar loading by including a patellar force transducer while measuring kinematics In one study, the anterior-posterior position of the femur on the tibia was examined and direct comparisons were made based on the status of the PCL [4].The specific conditions that were studied included the intact PCL, the loose PCL, and a PCL-substituting design Additionally, the post position was varied from mm more anterior to mm more posterior These implant conditions were shown to directly affect the anterior-posterior position of the femur on the tibia (⊡ Fig 23-6a) Simultaneous measurement of patellofemoral contact forces demonstrated about 20% lower forces for the most posterior femoral position (⊡ Fig 23-6b) Another in vitro study by D’Lima et al also supports these findings that a more posterior femoral position (roll-back) significantly decreases patellar compressive forces [6] This occurs in a normal knee with an intact PCL or with a posterior-stabilized implant but is less likely in a PCL-retaining design, especially when the PCL is loose or deficient In vitro setups have also been used to study patellar preparation in TKA [20].Comparisons were made for different patellar prosthesis designs (resurfacing vs inset) When measuring anterior patellar bone strain,it was concluded that a patellar resurfacing design is superior (less alteration to strain) to an inset design and that the osteotomy for patellar resurfacing is more tolerant to error by excess cutting than is the reaming technique required for an inset design.Another finding of the study was that if the ideal depth of the cut or reaming is surpassed (making the patella too thin), attempts to recreate the original patellar thickness by using a thicker prosthesis are mechanically detrimental 157 Chapter 23 · In Vitro Kinematics of the Replaced Knee – S Incavo, B Beynnon, K Coughlin Posterior PCL substituting TKA, post positioned posteriorly FR (mm) PCL substituting TKA, standard post position -2 Anterior PCL subsituting TKA, post positioned anteriorly PCL retaining TKA, with PCL intact -4 PCL sacrificing TKA, with PCL resected -6 20 a 40 60 Flexion Angle (degrees) 80 100 PCL sacrificing TKA, PCL resected PCL retaining TKA, PCL intact PCL substituting TKA, post positioned posteriorly PCL substituting TKA, standard post position PCL substituting TKA, post positioned anteriorly 800 600 PL (N) 400 200 0 b 20 40 60 Flexion Angle (degrees) 80 100 ⊡ Fig 23-6a, b a Average difference in the femoral roll-back (■ FR) versus flexion ■ FR is the difference in femoral roll-back between the treatment of interest and the reference (standard post position) treatment b Average patellofemoral contact load (PL) versus flexion (From [4]) Conclusions Knee kinematics and TKA design have been successfully studied using in vitro methods and represent useful complements to in vivo studies In particular, in vivo studies can examine individual cases, while in vitro studies are most useful to study the same specimen with a variety of different treatment options References Blaha JD et al (2003) Kinematics of the human knee using an open chain cadaver model Clin Orthop 410:25-34 Blankevoort L et al (1990) Helical axes of passive knee joint motions J Biomech 23:1219-1229 Churchill DL et al (1998) The transepicondylar axis approximates the optimal flexion axis of the knee Clin Orthop 356:111-118 Churchill DL et al (2001) The influence of femoral roll-back on patellofemoral contact loads in total knee arthroplasty J Arthroplasty 16:909-918 Coughlin KM et al (2003) Tibial axis and patellar position relative to the femoral epicondylar axis during squatting J Arthroplasty 18:10481055 23 158 23 III Kinematics D’Lima DD et al (2003) Impact of patellofemoral design on patellofemoral forces and polyethylene stresses J Bone Joint Surg [Am] 85 [Suppl 4]: 85-93 D’Lima DD et al (2000) Comparison between the kinematics of fixed and rotating bearing knee prostheses Clin Orthop 380:151-157 Elias S et al (1990) A correlative study of the geometry and anatomy of the distal femur Clin Orthop 260:98-103 Frankel V (1971) Biomechanics of the knee Orthop Clin North Am 2:175190 10 Hollister AM et al (1993) The axes of rotation of the knee Clin Orthop 290:259-268 11 Incavo SJ et al (1997) Knee kinematics in genesis total knee arthroplasty A comparison of different tibial designs with and without posterior cruciate substitution in cadaveric specimens Am J Knee Surg 10:209-215 12 Incavo SJ et al (2003) Anatomic rotational relationships of the proximal tibia, distal femur, and patella: implications for rotational alignment in total knee arthroplasty J Arthroplasty 18:643-648 13 Jonsson H et al (1994) Three-dimensional knee joint movements during a step-up: evaluation after anterior cruciate ligament rupture J Orthop Res 12:769-779 14 Karrholm J et al (1994) Kinematics of successful knee prostheses during weight-bearing: three-dimensional movements and positions of screw axes in the Tricon-M and Miller-Galante designs Knee Surg Sports Traumatol Arthrosc 2:50-59 15 Li G et al (2001) Cruciate-retaining and cruciate-substituting total knee arthroplasty: an in vitro comparison of the kinematics under muscle loads J Arthroplasty 16 [Suppl 1]:150-156 16 Most E et al (2003) The kinematics of fixed- and mobile-bearing total knee arthroplasty Clin Orthop 416:197-207 17 Nordin M, Frankel VH (2001) Biomechanics of the knee In: Nordin M, Frankel VH (eds) Basic biomechanics of the musculoskeletal system Lippencott Williams and Wilkins, Philadelphia 18 Weidenhielm L et al (1993) Knee motion after tibial osteotomy for arthhrosis Acta Orthop Scand 64:317-319 19 Woltring H et al (1985) Finite centroid and helical axis estimation from noisy landmark measurements in the study of human joint kinematics J Biomech 18:379-389 20 Wulff W, Incavo SJ (2000) The effect of patella preparation for total knee arthroplasty on patellar strain: a comparison of resurfacing versus inset implants J Arthroplasty 15:778-782 159 Chapter 24 · The Virtual Knee – B W McKinnon, J K Otto, S McGuan 24 24 24 The Virtual Knee B W McKinnon, J K Otto, S McGuan Summary The Virtual Knee is a 3-D, dynamic, physics-based software that simulates in vivo functional activities for the purpose of evaluating the kinematic and kinetic performance of TKR designs Implant models are virtually implanted onto a lower-leg purdue-like knee simulator that is driven through activities including gait and deep knee bend using active quadriceps and hamstring actuators The surrounding soft tissues, including LCL, MCL, and capsule, are modeled By varying parameters such as implant geometry, ligament tensions, component positioning, and patient anthropometrics, this complex system can be understood,which allows the design of better-performing implants Drawings must be prepared and tooling and gaging must all be designed and manufactured just to produce a highfidelity prototype implant.The physical testing machines for TKR performance evaluation include servohydraulic machines (e.g., MTS and Instron), wear simulators [1, 2], and Oxford/Purdue-type knee simulators [3, 4] But these machines have their shortcomings They often generate insufficient data because inline force transducers provide only force magnitude and not direction, multi-degree of freedom transducers are large and expensive, and multiple transducers add complexity Some testing machines oversimplify the real-world conditions Often derived resultant forces are applied to the implants, leaving out major stability-contributing tissues (e.g., quadriceps, hamstrings, collateral ligaments, and capsule) Also, the nonlinear properties of these tissues are hard to replicate and Introduction The typical engineering design process for most complex mechanical systems is illustrated in ⊡ Fig 24-1 This process is followed by many industries including the aerospace, the automotive, and of course the orthopedic industry In this process, virtual testing and physical testing design-iteration loops are employed, both of which are important methods for arriving at a final design that meets the design inputs The virtual testing loop uses computational methods to evaluate performance before physical components are ever made Although virtual methods cannot perfectly model the physical world, the advantages of this method are that many design iterations can be evaluated quickly and cheaply Moreover, all variables are controlled, and quality measurements can be extracted for nearly everything modeled in the system.Virtual testing is a powerful method for characterizing the system and understanding how specifically varied parameters affect the results With the exception of finite element analysis, the orthopedic industry has generally been without a robust, dynamic analytical testing method Consequently, total knee replacement design has relied heavily on physical testing methods Although the physical testing loop measures realworld performance, it is time consuming and expensive ⊡ Fig 24-1 Engineering design process for complex systems 160 24 III Kinematics they are difficult to attach to testing fixtures The simulations are generally limited to less demanding activities like walking, stair climbing, and deep knee bend because of inertia, fixture interference, and limited actuator stroke More demanding activities (e.g., running, tennis, and skiing) are too challenging to replicate.When cadavers are used, they are variable in size, shape, and location of anatomical landmarks In addition, great care must be taken to minimize implantation alignment errors Because of high cost and long lead times, the number of physical design iterations is severely limited In addition, the confounding factors of the physical testing machines make it difficult to interpret the results and make educated decisions about design changes.As a result,TKR design has evolved slowly, and the kinematic and kinetic performance has not been optimized Recently, an analytical tool called the Virtual Knee (Biomechanics Research Group, San Clemente, Calif., USA) has been used to address the shortcomings of the physical testing phase, and it has the potential to greatly advance TKR design Materials and Methods The Virtual Knee is a 3-D, dynamic, physics-based software that simulates in vivo functional activities for the purpose of evaluating the kinematic and kinetic performance of TKR designs Every parameter during the simulation is controlled and can be kept identical between trials.By comparing the results between design iterations, changes in kinematics and kinetics can be directly attributed to changes in the articular geometry, allowing designers to more easily meet performance design goals The Virtual Knee models a lower limb mounted to a Purdue-like knee testing rig (⊡ Fig 24-2).Anatomically accurate 3-D bone models of the femur, tibia, and patella with the desired anthropometrics are mounted to the rig and serve as reference for implant placement, joint line position, scale, and ligament/muscle attachment The constraints imparted on the simulation include the hip and ankle joint, passive soft tissues, intrinsic geometric contact constraint, and active muscle elements The hip joint is modeled as a revolute joint, parallel to the flexion axis of the knee, and is allowed to slide vertically The ankle joint is modeled as a combination of several joints that combine to allow free translation in the ML direction and free rotation in flexion, axial, and varus/valgus directions Passive tissue constraints are modeled as spring/damper elements and are attached to the virtual bones at their respective anatomical locations [5] The mechanical properties of the tissues were obtained from Woo [6] The LCL is simulated with a single constraint element, and the MCL is modeled with two separate constraint elements to simulate the anterior and posterior ⊡ Fig 24-2 The Virtual Knee simulates lower leg in vivo functional activities for the purpose of evaluating the kinematic and kinetic performance of TKR designs fibers (⊡ Fig 24-3).For posterior cruciate-retaining knees, a PCL element is added and attached at the respective anatomical locations The final soft-tissue constraint is the general capsule force, which simulates the general soft-tissue reactions of the knee capsule This force is directed such that it draws the femur and tibia together in a similar manner to the capsular tissues in the actual knee Intrinsic geometric constraints are imparted by the conformity of the TKR models, which includes stick/slip friction and stiffness characteristics.A contact algorithm models the articular surfaces, which are discretized into quadrilateral elements, as a bed of springs with the stiffness characteristics of polyethylene The method allows for intermittent contact, contact pressure, and center of pressure determination This algorithm is used to model contact of the patellofemoral joint, the tibiofemoral joint, between the cam and post, and between the quadriceps tendon and femoral component The active, driving elements in the model are the quadriceps and hamstrings muscle forces The quadriceps muscle attaches to the quadriceps tendon and is discretized into six components which conform to the distal head of the femur or anterior flange of the femoral component, permitting proper force transmission to the femur and patella The patella is attached to the tibial tubercle through the patellar ligament, which conforms to the polyethylene insert component.The simulation is driven by a controlled actuator arrangement similar to the 161 Chapter 24 · The Virtual Knee – B W McKinnon, J K Otto, S McGuan ⊡ Fig 24-3 Bones, passive soft tissues, active muscles, and component contact are modeled physical machine A closed-loop controller is used to apply tension to the quadriceps and hamstring muscles to match a prescribed knee flexion vs time profile No large antagonistic forces are modeled Ground reaction forces are applied as varus/valgus forces and internal/external torques during the cycle using time history data derived from force plate experiments [7] Two main activities are simulated, a complete cycle gait, and a 0°-160°-0°-160° double deep knee bend cycle The double cycle is performed so as to capture the inertial loading conditions at full extension (the second 0°) Three different reference frames are utilized for the reporting of data These reference frames are rigidly attached to the femur, tibia, and patella at the respective interfaces where the implant models meet the bone models, so as to easily resolve the interface forces and moments In the simulation,most kinematic and kinetic data are reported relative to the reference frame fixed in the tibia Kinematic and kinetic data for the patella are also reported with respect to the femur Component orientation is reported using a three-cylindric model of knee motion similar to Grood and Suntay [8] Data are reported via graphical animations and numerical results The graphical animations serve as a powerful communicative tool to design teams and surgeons Bones, muscles, soft tissues, and implants can all be selectively displayed during the animations The center-ofpressures for articular contact are displayed as spheres, and all contact forces and tissue forces are displayed as scaled force vectors (Fig 24-3) Currently, 84 data timehistories are reported for each simulation These include patellofemoral and tibiofemoral kinematics, soft-tissue forces and locations, actuator forces and locations, contact forces and center-of-pressure locations, contact area, interface forces and locations, and all externally applied forces and locations All the data are post-processed in a custom in-house spreadsheet, allowing graphical comparative analysis between trials The Virtual Knee has been validated in a variety of ways including mechanical,cadaver,and live subject tests Mechanical tests have been used to tune the performance of the contact force algorithm for both the shear component (friction/stiction) of the force and the normal component of the force The shear component of the contact force was tuned by comparing the Instron test machine results for the ASTM 1223 component laxity test with a virtual model of the laxity test The normal component of the forces was tuned by comparing the resulting contact area of virtual compression tests with the contact areas reported from Instron-Fuji film tests.With the current set of contact parameters, the contact algorithm has consistently delivered results within 10% of mechanical tests [9] Cadaver tests have been used to tune the soft tissues (attachment locations and mechanical properties) and the system controller function comparing the virtual rig results with physical machine results Human tests have been used to compare kinematic trends between the virtual simulation and in vivo data derived from fluoroscopy [10] Discussion Analytical functional simulation tools like the Virtual Knee provide key information that engineers and surgeons need to enable the use of several other powerful analytical tools and methods to design advanced TKR systems These tools include statistical methods such as sensitivity analysis, response surface methodology, and design of experiments (widely used today to study, control, and optimize manufacturing systems) Through functional simulation, designers can now define, isolate, 24 162 III Kinematics 24 ⊡ Fig 24-4 Full-body muscle-driven skeletal model for evaluation of a wide variety of activities and control the factors affecting the TKR system, analyze the factors using these statistical tools, and subsequently understand how to manipulate the factors to optimize performance Factors of particular interest to surgeons are those that affect stability and performance (and which can be variably controlled during simulation) including ligament tension, muscle strength, component alignment, surgical technique, patient anthropometric variability, implant selection, size mismatch, and material selection Future analytical functional simulation of TKR systems could involve the use of full-body muscle-driven simulations (⊡ Fig 24-4) rather than the single-leg simulation of the Virtual Knee In these simulations, motion capture data drive the skeletal model in displacement control, and the muscles are “trained” to perform the activity.After applying forward dynamics analysis,the muscles then drive the motion, and implant performance is evaluated These simulations are not limited to the standard gait,stair climbing,and deep knee bend movements, but instead allow for more demanding activities With the addition of other active and passive tissues, normal knee kinematics and kinetics could be compared with those of the replaced knee for various functional activities, possibly defining new TKR performance testing methods and measures that are more closely correlated to in vivo performance Furthermore, the Virtual Knee does not have to be limited to TKR design Patellofemoral and unicondylar designs could all benefit In addition, awareness of implantation alignment sensitivities can also drive improved instrument design The current state of the art in computer modeling,analytical simulation, and statistical analysis offers exciting opportunities for the designing surgeon and engineer.All of the key ingredients now exist to enable a revolution in TKR design.Parametric 3-D CAD models of implants can now be precisely controlled and quickly manipulated in a manner that allows for optimization of the geometry Reverse-engineered CT and MRI knee scans can provide the geometry required to optimize implant shape, fit, and size Analytical tools such as the Virtual Knee can accurately model knee function,provide critical measures not possible with physical testing, and reduce cost and lead times The resulting kinematics measured from the Virtual Knee can be used to drive dynamic finite element analyses [11], providing an enhanced picture of polyethylene stresses,which may be used as a predictor of in vivo wear performance [12].Finally,proven statistical methods can be used to effectively consider the many factors affecting TKR performance,allowing optimization of those factors to create the desired performance envelope.In the end, the patient will benefit the most by receiving thoroughly tested and optimized knee replacements implanted by surgeons who are more aware of the factors that most affect the desired results References Walker PS et al (1997) A knee simulating machine for performance evaluation of total knee replacements J Biomech 30:83-89 Burgess IC et al (1997) Development of a six-station knee wear simulator and preliminary wear results Proc Inst Mech Eng [H] 211:37-47 Biden E, O’Connor J (1990) Knee ligaments: structure, function, injury, and repair Raven, New York Zachman NJ (1977) Design of a load simulator for the dynamic evaluation of prosthetic knee joints MS Thesis Mechanical Engineering, Purdue University, West Lafayette, IN Blankevoort L, Huiskes R (1991) Ligament-bone interaction in a three-dimensional model of the knee J Biomech Eng 113:263-269 Woo SL et al (1991) Tensile properties of the human femur-anterior cruciate ligament-tibia complex The effects of specimen age and orientation Am J Sports Med 19:217-225 Winter D (1990) Biomechanics and motor control of human movement Wiley-Interscience, New York Grood ES, Suntay WJ (1983) A joint coordinate system for the clinical description of three-dimensional motions: application to the knee J Biomech Eng 105:136-144 Masson M et al (1996) Computer modeling of articular contact for assessing total knee replacement constraint criteria 10th Conference of the European Society of Biomechanics, Leuven 10 Banks SA et al (1997) The mechanics of knee replacements during gait In vivo fluoroscopic analysis of two designs Am J Knee Surg 10:261-267 11 Godest AC et al (2002) Simulation of a knee joint replacement during a gait cycle using explicit finite element analysis J Biomech 35:267-275 12 Fregly BJ et al (2003) Computational prediction of in vivo wear in total knee replacements Proc 2003 Summer Bioengineering Conference, American Society of Mechanical Engineers, New York IV Surgical Technique 25 Optimizing Alignment – 165 M A Rauh, W M Mihalko, K A Krackow 26 Assess and Release the Tight Ligament – 170 L A Whiteside 27 The Technique of PCL Retention in Total Knee Arthroplasty – 177 T J Thornhill, T S Thornhill 28 Posterior Cruciate Ligament Balancing in Total Knee Arthroplasty with a Dynamic PCL Spacer – 182 A B Wymenga, B Christen, U Wehrli 29 Achieving Maximal Flexion – 188 J Bellemans 30 Assess and Achieve Maximal Extension – 194 R S Laskin, B Beksac 31 Understanding the Rheumatoid Knee – 198 K K Anbari, J P Garino 32 Management of Extra-Articular Deformities in Total Knee Arthroplasty – 205 K G Vince, V Bozic 33 Use of a Tensiometer at Total Knee Arthroplasty T J Wilton 34 Specific Issues in Surgical Techniques for Mobile-Bearing Designs – 217 P T Myers 35 Optimizing Cementing Technique G R Scuderi, H Clarke – 223 – 212 36 Assessment and Balancing of Patellar Tracking J H Lonner, R E Booth, Jr 37 Specific Issues in Surgical Techniques for Unicompartmental Knees – 234 L Pinczewski, D Kader, C Connolly – 228 165 25 25 25 Optimizing Alignment M A Rauh, W M Mihalko, K A Krackow Summary The History of Optimal Alignment Optimizing alignment in total knee arthroplasty requires an understanding of the assumptions of a chosen instrumentation system This understanding involves knowing the possible alignment errors of a system and knowing how the particular system leads the surgeon through the various steps of component placement Current intramedullary and extramedullary instrumentation can assist in component placement; however, the surgeon must be aware of situations such as extra-articular deformities that can affect the final alignment Additionally, computer-assisted knee navigation is available for routine use.These systems allow for more accurate positioning of jigs, the ability to correctly establish femoral component rotation, instantaneous feedback on overall alignment, and the ability to prevent implantation of malpositioned components Overall, computer assistance results in decreased variability and the elimination of outliers An armamentarium of alternative techniques must be kept in mind for use as secondary checks on the primary technique and in situations of distorted or absent anatomical landmarks A consensus exists on most aspects of “normal” knee alignment Such normal alignment involves issues of the proper mechanical axis and joint line orientation More specifically, proper alignment at the knee can be characterized by two independent conditions: The normal or prosthetic knee joint should be centered on the mechanical axis of the lower extremity “Proper” orientation of the joint line should exist Few dispute that the single most important element to successful and long-lasting total knee arthroplasty is accurate alignment of the implants [1-7] Introduction Optimizing total knee arthroplasty alignment requires an understanding of the assumptions of a chosen instrumentation system This understanding involves knowing the possible alignment errors of a system and knowing how the particular system leads the surgeon through the various steps of component placement Current intramedullary and extramedullary instrumentation can assist in component placement; however, the surgeon must be aware of situations such as extra-articular deformities that can affect the final alignment An armamentarium of alternative techniques must be kept in mind for use as secondary checks on the primary technique and in situations of distorted or absent anatomical landmarks Historically,instrumentation systems have been grouped under two headings, referred to here as classical alignment and anatomical alignment [8] In the classical, most common variety, the theoretically correct goal is the establishment of a joint line perpendicular to the reconstituted mechanical axis As a result, the proximal tibial cut is perpendicular to the overall tibial shaft axis; and since the distal femoral cut is perpendicular to the femoral portion of the mechanical axis, it is oriented at an angle β, approximately 6° relative to the femoral shaft axis (⊡ Fig 25-1) ⊡ Fig 25-1 Variation in alignment from the mechanical to the anatomical axes of the femur 183 Chapter 28 · Posterior Cruciate Ligament Balancing in Total Knee Arthroplasty – A B Wymenga et al with a tensor in the knee joint exerting 150-200 N tension An intramedullary femur guide is used to indicate the direction of the femur cut Following the releases in extension,the tibia cut should be parallel to the plane of the distal femur cut This indicates a correct mechanical axis, and no further releases are performed Thereafter the knee is flexed and a double tensor is inserted with 100-150 N The femur finds its rotational position through the ligament tension of the central PCL and the collateral ligaments The femur cutting guide is inserted over an intramedullary rod and rotated parallel to the tibia cut, creating a rectangular flexion space The anterior and posterior femur osteotomies are performed The size of the femoral component is matched with the original medial femoral condyle dimensions, and approximately mm (prosthesis thickness) is cut from the posterior femoral condyle.This leaves the joint line on the medial side of the knee at the same level Now the PCL is balanced with the BalanSys PCL Tensioner with an adjustable spacer block On the adjustable spacer the corresponding polyethylene sizes are marked 8,10.5,13,and 15.5 mm.Thickness of the tibial tray (2 mm) and femoral component (9 mm) of the knee system are included When the PCL tensioner is opened the PCL spacer increases in size.The surgeon can read on the handle of the device how much tension is being applied (scale from to 250 N) On the femur side a sliding plateau is mounted which enables anterior tibia translation when the spacer is gradually opened Since the fiber course of the PCL is oblique from tibia inferior-posterior to the medial femoral condyle superior-anterior,the opening of the joint space with a tensor causes the tibia to move forward when the PCL is tensioned (⊡ Figs 28-1, 28-2) At the beginning of the tensioning by the BalanSys PCL Tensioner the joint opens in a proximal-distal direction without translation as the PCL is not tensioned As soon as the PCL is tensioned, the oblique fibers will pull the tibia forward We currently use a 2- to 3-mm anterior translation for the correct pretension of the PCL The relative positions of femur and tibia with the spacer in situ are accepted as the correct position Now the surgeon has to check whether the prosthesis can be fitted into the created flexion space ▬ If the indicated PE thickness on the BalanSys PCL Tensioner is mm, this is accepted ▬ If the flexion space is too small (e.g., mm PE) the flexion space should be enlarged The surgeon can choose to make an additional tibia cut of mm or to use a smaller femur size (increments of mm) After this additional cut is made, the flexion space can harbor the tibia tray, the mm PE, and the femur component and, given the volume of the prosthesis material, the PCL is automatically pretensioned after implantation of the material a b c d ⊡ Fig 28-1a-d Tensioning the PCL during surgery with the BalanSys PCL Tensioner a Without tension; b with tension In c the anterior translation can be read, and in d the gap size can be read 28 184 IV Surgical Technique can only downsize the femur in order to gain more flexion space since a tibia re-cut would cause instability in extension In case of a loose flexion space a larger polyethylene size can be used with a re-cut of the distal femur Results a Flexion Gap Dynamics Measured with the BalanSys PCL Tensioner b ⊡ Fig 28-2a, b Orientation of the PCL and relative movements of the femur onto the tibia As the flexion gap increases by applied tension, the tibia is pulled anteriorly by the PCL After the PCL balancing is finished the extension cut on the distal femur is made, guided by a tensor with 150 N and anticipating the same polyethylene thickness as chosen in flexion If a bone-referenced technique is used and all bone cuts are made before flexion-space testing, the adjustment possibilities are more limited With a tight flexion space and a correct extension space the surgeon ⊡ Table 28-1 Relation of flexion gap, tension, and anterior tibial translation Tension BalanSys PCL tensioner 100 N 150 N 200 N Flexion gap size in mm1 (SD, range) Anterior translation in mm (SD, range) 6.3 (0.6, 5.5-10.0) 0.6 (0.6, 0.0-2.5) 7.6 (1.1, 5.0-11.0) 2.2 (1.7, 0.0-8.5) 9.5 (1.8, 6.5-14.0) 4.3 (2.0, 0.0-10.0) Flexion gap size expressed in mm polyethylene; for real flexion gap size mm tibia tray and mm femoral component thickness should be added 16,0 14,0 12,0 Flexion Gap size 28 ▬ If the flexion space is larger than mm the surgeon can choose a larger PE size If the flexion space size is between two PE sizes (e.g., mm) a larger size (10.5 mm) can be chosen but an additional 1.5-mm bone cut has to be made In a prospective study at the Department of Orthopedic Surgery, Spital Bern Ziegler, 82 patients received a total knee arthroplasty with a PCL-sparing technique.The BalanSys PCL Tensioner was used to balance the PCL and create the correct flexion space The size of the flexion gap was measured with 100 N, 150 N, and 200 N Also the anterior translation of the tibia was measured with the spacer The results are summarized in ⊡ Table 28-1 There is a large variation in the amount of opening of the flexion space and the applied tension In some patients the flexion space opens up to 14 mm with 200 N, whereas in others the joint space is only 6.5 mm.This may depend on the amount of tibia resection, the pre-existing laxity and morphotype of the patient, and the angulation of the PCL fibers, which may run a more vertical or horizontal course in some patients Also, the weight of the extremity has an influence (⊡ Figure 28-3) y = 0,5046x + 7,3216 10,0 8,0 6,0 4,0 2,0 0,0 0,0 2,0 4,0 6,0 Anterior translation 8,0 10,0 12,0 ⊡ Fig 28-3 Relation between the increase of the flexion gap (mm) and anterior translation (mm) of the tibia with use of the BalanSys PCL Tensioner measured from to 200 N tension 28 185 Chapter 28 · Posterior Cruciate Ligament Balancing in Total Knee Arthroplasty – A B Wymenga et al More important, however, is the anterior translation We found on average a 1:2 ratio between the increase of the joint space and the anterior translation when the tension was increased from to to 200 N There was some variation from knees having a 1:1.5 ratio to a more than 1:2 ratio,as can be seen in Fig.28-2.From these data it is clear that the flexion space of a knee with an intact PCL is not defined as a fixed space but rather as a space depending on the tension and the direction of the PCL fibers and the anteroposterior position of the tibia The greater the PCL tension, the larger the space becomes and the more anterior translation of the tibia occurs It is also clear that small adjustments of the flexion space have a significant influence on the anteroposterior position of the femur If, for instance, an additional 3-mm bone cut of the tibia is made, the contact position of the femur will change (based on the 1:2 ratio) × = mm, which can be the difference between a too posterior and a correct contact position of the femur For example, if a 2.5-mm-larger PE insert is used, the femur will move mm posteriorly, which could make the knee too tight Anteroposterior Laxity Measured in 90° of Flexion in Clinical Patients with a TKA Implanted with the PCL Tensioner In a prospective study (results prepared for publication) at the Department of Orthopedic Surgery, Spital Bern Ziegler, 141 patients (37 male and 104 female) were treated with a BalanSys TKA between May 2001 and May 2003 Mean age of the patients was 71 years (49-89).All patients were treated for osteoarthrosis In 54 patients with valgus knees a lateral approach and in 87 patients with a varus knee a medial approach was used These patients were evaluated with the Rolimeter (Aircast Europe Ltd.) [9-12] ⊡ Table 28-2 Anteroposterior laxity measured with rolimeter in 90° of flexion Measure Preoperative Postoperative Median Mean SD Range 6.7 7.0 2.9 0-17 5.0 5.2 2.0 2-10 in order to measure anteroposterior laxity of the knee in 90° of flexion preoperatively and postoperatively in the operating room after implantation of the prosthesis The results are shown in ⊡ Table 28-2 In a subset of these patients (n=65) the average flexion after surgery was 117.5°, indicating that with an adequate PCL, balancing can give a very stable knee in flexion with an anteroposterior drawer of only mm and, despite this, a good range of motion Kinematics of TKA with the BalanSys PCL Tensioner in Laboratory Experiments In a laboratory experiment five TKAs were implanted in fresh-frozen knee specimens using the BalanSys PCL Tensioner (results prepared for publication) The procedures were supported by a navigation system (PRAXIM, Grenoble, France) A relatively conforming, fixed-bearing insert was tested and then, in the same specimen, an AP-gliding meniscal-bearing insert was tested After recording of the knee kinematics with passive motion, the PCL was cut and kinematics were recorded with a non-functional PCL.To quantify AP translation of the femur relative to the tibia (results in Fig 28-4), the reference point on the femur was chosen as the intersection 4,0 2,0 Anteroposterior position 0,0 ⊡ Fig 28-4 Average (n=5, mean +/-1 SEM) femoral AP translation of five knee specimens with a mobile- (MB) and a fixed-bearing (FB) TKP both before and after resection of the PCL 15 30 45 60 75 -2,0 -4,0 -6,0 -8,0 -10,0 FB with PCL FB without MB with MB without -12,0 Flexion (degrees) 90 105 120 186 IV Surgical Technique of the femur rotation axis (femoral component: single radius design) and a central sagittal plane through the femoral component The fixed- and meniscal-bearing configurations show a correct contact point posterior to the middle of the tibial plateau in AP direction, even in extension (⊡ Fig 28-4) In extension the meniscal variant is mm anterior compared with the fixed-bearing variant In flexion both bearing types resulted in roll-back between 60° and 120° of flexion with a correct contact point similar to the normal knee in 90° of flexion [6] After the PCL has been cut the kinematics are disturbed and the femur contact point moves forward The effects for the fixed-bearing insert are limited, whereas the effects for the AP-gliding meniscal-bearing insert are quite considerable These data illustrate the effectiveness of PCL balancing with the BalanSys PCL Tensioner and confirm the importance of a well-functioning PCL, especially in AP-gliding meniscal-bearing knees 28 Discussion A new concept of flexion-gap balancing with the BalanSys PCL Tensioner was developed With the use of this dynamic spacer,the PCL can be adequately tensioned; the required amount of flexion gap for the prosthesis can be precisely determined and, if necessary, adjusted with small bone cuts or an increase of PE size Patients from a case series operated on with this technique had an average of 117.5° of flexion with normal anteroposterior laxity, indicating a functional PCL The flexion gap is dependent on PCL tension and the anterior translation of the tibia A small increase of the flexion space causes a comparatively large anterior translation of the tibia with a ratio of 1:2 This is caused by the obliquely oriented PCL fibers that pull the tibia anterior with the opening of the flexion gap.A few millimeters difference of bone resection can change the contact point of the femur on the tibia considerably If the flexion gap is accepted without sufficient tension of the PCL the femur automatically slides forward If the PCL is too tight the contact position is too posterior These findings may explain why it was difficult in some laboratory experiments to achieve normal PCL function with a PCL-retaining TKA [13] Mahoney et al [14] found a normal PCL strain in only 37% after TKA They also concluded that the PCL strain was increased by 50% by inserting a 2.5-mm-thicker polyethylene insert Most [15] and Sorger [16], however, were able to balance PCL and the flexion gap in laboratory tests and to restore normal ‘roll-back’ Li [17] found a partial restoration of the roll-back beyond 60° of flexion If the PCL was cut he found reciprocal anterior translation, showing the importance of the PCL Also in clinical series with fixed-bearing knees variable results are found The PCL function can be analyzed by measuring the contact point of the femur on the tibia in flexion and AP-laxity in flexion Misra [18] compared PCL resection and retention with a fixed-bearing condylar prosthesis and found roll-back in only 20% of both groups, indicating that PCL balancing was not achieved Matsuda [19] achieved only good anteroposterior stability in half of the patients with a PCL-retaining prosthesis Dejour [20] found anteroposterior instability in 41% of the patients with posterior cruciate-retaining (PCR) TKA, indicating a non-functioning PCL Straw [21] found anteroposterior laxity in 54% of patients with PCR TKA Kim [22], however, was able to achieve a contact point of 55% with PCR TKA, which is similar to that in the normal knee [6] In mobile-bearing knees PCL balancing seems to be even more critical Morberg [3] found a high rate of failures after AP-gliding mobile-bearing total knee arthroplasty due to flexion instability caused by inadequate PCL balancing Archibeck [23] found a low rate of clinical posterior instability and measured an average contact point of 45% (22%-98%) in meniscal-bearing knees This contact point shows that the PCL balancing was not perfect in all patients Hartford [24] found anterior sliding of the meniscal bearing in 70% of patients,indicating a non-functional PCL; the patients with roll-back had better knee scores and better flexion Anteroposterior-gliding mobile bearings and flat fixed bearings depend more on soft-tissue restraints, whereas more fixed conforming designs are more self-stabilizing [25] Also with fluoroscopic analysis in vivo in PCR TKAs frequently show a paradoxical anterior sliding in flexion [26, 27], but results seem to vary between individual surgeons [26,28] Bertin found a consistent posterior contact point in PCR TKA [29].Better flexion was found in patients with a posterior contact position From these studies it is clear that PCL balancing and the creation of a normal contact point of the femur on the tibia are difficult to achieve in PCL-retaining TKA with a bone-referenced technique The flexion gap is not a static space but a dynamic space that is controlled mainly by the oblique PCL fibers that link flexion gap size and tibia translation in a 1:2 ratio With the newly developed dynamic PCL spacer it is possible to adequately balance the PCL in a reproducible way Conclusions A ligament-guided PCL-balancing technique was developed with a new device,the BalanSys PCL Tensioner.This dynamic PCL spacer controls the PCL tension and also determines exactly the required flexion space size Changes in flexion space have a large effect on anterior translation of the tibia, which may explain the difficulty 187 Chapter 28 · Posterior Cruciate Ligament Balancing in Total Knee Arthroplasty – A B Wymenga et al with PCL balancing in the past The flexion space is adjusted with small 1- to 2-mm bone cuts and the PCL is not released We were able to achieve a high rate of anteroposterior stability with an average ROM of 117.5° in patients operated on with this technique, indicating a functional PCL.Laboratory tests confirmed normal kinematics and normal contact points with the new PCLbalancing technique Acknowledgements We acknowledge the co-authorship of A J Schuster and S R Thomann (Department of Orthopedic Surgery, Spital Bern Ziegler, Switzerland), T Wyss (Balgrist Spital, Zurich, Switzerland) and W Jacobs (St Maartenskliniek, Nijmegen, The Netherlands) References Gill GS, Joshi AB, Mills DM (1999) Total condylar knee arthroplasty: 16- to 21-year results Clin Orthop 367:210-215 Waslewski GL, Marson BM, Benjamin JB (1998) Early, incapacitating instability of posterior cruciate ligament-retaining total knee arthroplasty J Arthroplasty 13:763-767 Morberg P, Chapman-Sheath P, Morris P, Cain S, Walsh WR (2002) The function of the posterior cruciate ligament in an anteroposterior-gliding rotating platform total knee arthroplasty J Arthroplasty 17:484-489 Pagnano MW, Hanssen AD, Lewallen DG, Stuart MJ (1998) Flexion instability after primary posterior cruciate retaining total knee arthroplasty Clin Orthop 356:39-46 Migaud H, Tirveilliot F (2003) Preservation, resection or substitution of the posterior cruciate ligament in total knee replacement In: Lemaire R, Horan F, Scott J, Villar R (eds) Proceedings of the EFORT congress European Instructional Course Lectures 6:176-184 Freeman MA, Pinskerova V (2003) The movement of the knee studied by magnetic resonance imaging Clin Orthop 410:35-43 Komistek RD, Dennis DA, Mahfouz M (2003) In vivo fluoroscopic analysis of the normal human knee Clin Orthop 410:69-81 Gollehon DL, Torzilli PA, Warren RF (1987) The role of the posterolateral and cruciate ligaments in the stability of the human knee A biomechanical study J Bone Joint Surg [Am] 69:233-242 Balasch H, Schiller M, Friebel H, et al (1999) Evaluation of anterior kneejoint instability with the rolimeter Knee Surg Sports Traumatol Arthrosc 7:204-208 10 Ganko A, Engebretsen L, Ozer H (2000) The rolimeter: A new arthrometer compared with Kt 1000 Knee Surg Sports Traumatol Arthrosc 8:36-39 11 Pässler H, Ververidis A, Monauni F (1998) Beweglichkeitswertung an Knieen mit Vkb-Schaden mit Hilfe des Kt 1000 und Aircast Rolimeter Unfallchirurg 272:731-732 12 Schuster AJ, McNicholas MJ, Wachtl SW, McGurty DW, Jakob RP (2004) A new mechanical testing device for measuring anteroposterior knee laxity Am J Sports Med (in press) 13 Corces A (1989) Strain characteristics of the posterior cruciate ligament in total knee arthroplasty Orthop Trans 13:527-528 14 Mahoney OM, Noble PC, Rhoads DD, Alexander JW, Tullos HS (1994) Posterior cruciate function following total knee arthroplasty A biomechanical study J Arthroplasty 9:569-578 15 Most E, Zayontz S, Li G, Otterberg E, Sabbag K, Rubash HE (2003) Femoral rollback after cruciate-retaining and stabilizing total knee arthroplasty Clin Orthop 410:101-113 16 Sorger JI, Federle D, Kirk PG, Grood E, Cochran J, Levy M (1997) The posterior cruciate ligament in total knee arthroplasty J Arthroplasty 12:869879 17 Li G, Zayontz S, Most E, Otterberg E, Sabbag K, Rubash HE (2001) Cruciate-retaining and cruciate-substituting total knee arthroplasty: An in vitro comparison of the kinematics under muscle loads J Arthroplasty 16 [Suppl 1]: 150-156 18 Misra AN, Hussain MR, Fiddian NJ, Newton G (2003) The role of the posterior cruciate ligament in total knee replacement J Bone Joint Surg [Br] 85:389-392 19 Matsuda S, Miura H, Nagamine R, Urabe K, Matsunobu T, Iwamoto Y (1999) Knee stability in posterior cruciate ligament retaining total knee arthroplasty Clin Orthop 366:169-173 20 Dejour D, Deschamps G, Garotta L, Dejour H (1999) Laxity in posterior cruciate sparing and posterior stabilized total knee prostheses Clin Orthop 364:182-193 21 Straw R, Kulkarni S, Attfield S, Wilton TJ (2003) Posterior cruciate ligament at total knee replacement: essential, beneficial or a hindrance? J Bone Joint Surg [Br] 85:671-674 22 Kim H, Pelker RR, Gibson DH, Irving JF, Lynch JK (1997) Rollback in posterior cruciate ligament-retaining total knee arthroplasty A radiographic analysis J Arthroplasty 12:553-561 23 Archibeck MJ, Berger RA, Barden RM, Jacobs JJ, Sheinkop MB, Rosenberg AG, Galante JO (2001) Posterior cruciate ligament-retaining total knee arthroplasty in patients with rheumatoid arthritis J Bone Joint Surg [Am] 83:1231-1236 24 Hartford JM, Banit D, Hall K, Kaufer H (2001) Radiographic analysis of low contact stress meniscal bearing total knee replacements J Bone Joint Surg [Am] 83:229-234 25 Walker PS, Ambarek MS, Morris JR, Olanlokun K, Cobb A (1995) Anteriorposterior stability in partially conforming condylar knee replacement Clin Orthop 310:87-97 26 Dennis DA, Komistek RD, Mahfouz MR, Haas BD, Stiehl JB (2003) Multicenter determination of in vivo kinematics after total knee arthroplasty Clin Orthop 416:37-57 27 Banks S, Bellemans J, Nozaki H, Whiteside LA, Harman M, Hodge WA (2003) Knee motions during maximum flexion in fixed- and mobile-bearing arthroplasties Clin Orthop 410:131-138 28 Nozaki H, Banks SA, Suguro T, Hodge WA (2002) Observations of femoral rollback in cruciate-retaining knee arthroplasty Clin Orthop 404:308-314 29 Bertin KC, Komistek RD, Dennis DA, Hoff WA, Anderson DT, Langer T (2002) In vivo determination of posterior femoral rollback for subjects having a Nexgen posterior cruciate-retaining total knee arthroplasty J Arthroplasty 17:1040-1048 28 29 29 Achieving Maximal Flexion J Bellemans Summary ⊡ Table 29-1 Most frequent reasons for limitation of flexion following TKA Despite all recent advances in total knee arthroplasty surgery, limited flexion remains a subject of frustration for many knee surgeons and patients Frequently, it is the physiotherapist, the patient, or the designing engineers who get the blame In our experience however, the most frequent reasons for limited knee flexion after TKA are surgical technical factors Extension-flexion gap balancing, adequate posterior clearance, and the avoidance of anterior overstuffing are indeed factors that can be controlled perfectly intraoperatively when recognized by the surgeon Failure to so will inevitably result in loss of flexion, despite the best physiotherapy, the most motivated patient, and the most optimal implant with regard to regaining flexion Introduction Many variables determine the final outcome after total knee arthroplasty Not all of these factors can be influenced by the surgeon; most are actually beyond the surgeon’s control The patient’s preoperative status, severity of the disease, associated co-morbidity, multiple joint involvement, motivational status, etc., are all important issues with great impact on the clinical and subjective results after TKA All this is equally true with regard to maximal flexion that is obtained by the patient after TKA [1-5] Nevertheless,there is growing evidence that concerning maximal postoperative flexion obtained by the patient, as well as design-related aspects of the implanted TKA system, surgical technical factors play an important role [6-11] In this chapter I will review the surgical and design-related factors which I consider to be most important in regaining flexion after TKA With regard to surgical technical factors, these can be subdivided into three important categories: (⊡ Table 29-1) The first category is related to inadequate flexion and extension gap balancing, with relative tightness of the flexion gap leading to potential overstuffing of the flexion space and mechanical blocking of further flexion Extension gap–flexion gap mismatch, due to Extension space >flexion space PCL too tight Excessive femoral component (internal or external) rotation Insufficient tibial slope Inadequate posterior clearance, due to Osteophytes Soft-tissue remnants Posterior tibial overhang Decreased posterior off-set Anterior overstuffing Patellar thickening Femoral or tibial anteriorization Retinacular tightness Quadriceps shortening Inadequate physiotherapy Inadequate implant The second group of technical factors concerns issues related to the posterior area of the knee joint The third group deals with issues related to the anterior aspect of the knee Together with increased awareness of these surgical factors, implant designers have recently shown increased attention towards implant design-related factors that may influence maximal range of motion postoperatively The renewed interest in the importance of femoral roll-back, as well as increased knowledge about the in vivo kinematic behavior of contemporary TKA systems,has played an important role in this process [8-10,12-17] This, together with an increased awareness of the physiological and anatomical characteristics that allow the normal knee to obtain maximal flexion, has led to the design and development of more optimal knee arthroplasty components with regard to maximizing postoperative flexion [6, 7, 10] Inadequate Flexion and Extension Space Balancing One of the most important issues in contemporary surgical TKA techniques is an adequate creation of the flexion 189 Chapter 29 · Achieving Maximal Flexion – J Bellemans ⊡ Fig 29-1 Equally sized flexion and extension spaces and extension spaces These spaces are defined as the opening of the knee joint space after completion of the bone cuts, while the knee is held in extension (extension space) or flexion (flexion space) These spaces are subsequently filled up by the prosthetic components, and their size is therefore critical,since a space that is too small may make it difficult to squeeze the components into the created space without causing excessive tension on the softtissue structures Likewise, when the space is too large, underfilling of the space by the prosthetic components will occur, with insufficient soft.tissue tension and instability as a result Since most contemporary knee arthroplasty designs have an identical metal thickness on their extension and flexion regions, the flexion and extension spaces created should therefore be equal in size (⊡ Fig 29-1) Moreover, they should be rectangular to allow for comparable tension in the medial and lateral soft tissues Some lateral laxity or wideness of the flexion and extension spaces may be accepted, however, since the normal knee is also a bit more lax on the lateral than on the medial side Despite the fact that the amount of bone resection is controlled by the surgeon,reality shows that unequal flexion and extension spaces are not seldom obtained during TKA surgery More specifically, if the flexion space is smaller than the extension space it may lead to problems with obtaining satisfactory postoperative flexion, if the situation is not addressed and corrected during surgery In theory, a thinner tibial insert could be used to avoid overstuffing the flexion space, but this would lead to underfilling of the extension space with inadequate soft-tissue tension to provide sufficient joint stability in extension However, several more appropriate options are available to the surgeon when attempting to correct this situ- ation of a tight flexion space.For example,downsizing the femoral component while maintaining the same anterior reference point will increase the flexion space without altering the extension space, and may correct the problem Increasing the tibial slope will also open up the flexion space for the prosthetic components,since the femorotibial contact points can move posteriorly during flexion During extension femorotibial contact occurs anterior and is therefore little influenced by increasing the tibial slope Finally, release of the posterior cruciate ligament (PCL) will predominantly increase the flexion space and is another way to compensate for flexion space tightness Such release can be performed by resecting the PCL,or by gradually releasing it from its bony attachments on the tibia or the femoral condyle When this option is chosen by the surgeon, a PCL-substituting implant should be used Apart from these general principles that can be applied in case of flexion space tightness, the surgeon should realize that a number of very specific factors may be the cause of the tightness PCL Shortening Shortening of the PCL is such a factor; it can be encountered in severe osteoarthritis or after previous high tibial osteotomy due to fibrotic scarring (⊡ Fig 29-2).PCL shortness can usually be diagnosed by two intraoperative signs: (a) the presence of excessive femoral roll-back during flexion testing with the trial components in situ, and (b) anterior liftoff of the tibial insert from the trial base plate during flexion testing It is obvious that in these situations,releasing the PCL is the method of choice for correcting the problem ⊡ Fig 29-2 Shortening of the PCL leads to tightness of the flexion space 29 190 IV Surgical Technique Femoral Component Malrotation Malrotation of the femoral component is another single variable that may be responsible for flexion space tightness It is well known that excessive internal rotation of the femoral component may cause lateral patellar tilting and subluxation, but it may also lead to asymmetric flexion space tightness at the medial side,leading to excessive tension and pain in the medial soft tissues when the knee is flexed (⊡ Fig 29-3) Likewise, excessive external femoral component rotation will lead to lateral flexion space tightness with excessive tension built up in the lateral soft tissues during flexion (⊡ Fig 29-4) Errors in femoral component rotation occur relatively frequently and are very difficult to correct or compensate for once the bone cuts have been made.Furthermore, the presence of femoral component malrotation may be 29 overlooked during surgery, since full passive flexion may still be possible in the anesthetized patient Once the patient is awake, however, pain in the excessively tensioned collateral soft tissues will prevent him or her from moving the knee into deep flexion The best way to avoid this problem is therefore to carefully assess the soft-tissue tensions during full range of motion, by digital palpation of the collateral structures and by varus or valgus stress testing with the knee in flexion In the case of excessive femoral component internal rotation, release of the anterior fibers of the medial collateral ligament from the tibial insertion can be performed in an attempt to improve the situation, since releasing these fibers will open the medial flexion space without influencing the extension space [18] Likewise, excessive femoral component external rotation can be addressed by releasing the popliteus tendon and lateral collateral ligament, leading to opening of the lateral flexion space If the iliotibial band is left intact, such release will not influence the extension space [18] In cases of very severe femoral malrotation, however, it is obvious that revising the bone cuts to the correct rotation will be required.The use of wedges or cement is frequently necessary in these cases in order to maintain implant contact with the revised bone cuts Insufficient Tibial Slope ⊡ Fig 29-3 Excessive femoral component internal rotation, leading to medial tightness of the flexion space ⊡ Fig 29-4 Excessive femoral component external rotation, leading to lateral tightness of the flexion space Most people consider the degree of tibial slope an important factor in maximizing postoperative flexion after TKA, despite the fact that few hard data are available to substantiate this The introduction of more conforming knee designs in particular has led to the general belief that some down-sloping of the tibial component is advisable, since it avoids the so-called kinematic conflict that is present during knee flexion, where femoral roll-back is prohibited by the up-sloping posterior lip of the tibial insert Down-sloping the tibial component may therefore reduce such conflict, and may allow the knee to go into deeper flexion Recent work by our group has indeed demonstrated a beneficial effect of increasing tibial slope with regard to maximal flexion [11] In a cadaver simulation of previously obtained in vivo kinematics of contemporary PCLretaining TKA patients, an average gain of 1.7° flexion for every degree of extra tibial slope was seen.In other words, increasing the slope by 6° leads to an average increase of flexion by 10° With regard to tibial slope, however, it is important to note that altering the slope also influences anterior stability of the knee [19].Excessive down-sloping may therefore lead to anterior instability and so-called mid-flexion laxity We therefore believe that the increase of the tibial slope should not exceed 7° 191 Chapter 29 · Achieving Maximal Flexion – J Bellemans Posterior Clearance The posterior aspect of the knee joint is crucial for the ability to regain maximal flexion following TKA Recent work by us has demonstrated that maximal obtainable flexion in patients with modern PCL-retaining TKA is ultimately determined by impingement of the tibial insert against the back of the femur [9, 10] (⊡ Fig 29-5) Such impingement obviously occurs faster when femoral osteophytes are not adequately removed,or when protruding bone beyond the femoral condyles is left behind (⊡ Fig 29-6) Likewise, soft-tissue remnants in the backside of the knee, such as meniscal remnants or hypertrophic synovial tissue, should be removed adequately for the same reasons According to the same principle, posterior tibial component overhang should be avoided Posterior Condylar Offset Restoring the posterior condylar offset is another important factor with respect to maximizing postoperative flexion In an analysis of 150 consecutive PCL-retaining TKA patients, we demonstrated a significant correlation between operative restoration of posterior condylar offset and maximal postoperative flexion For every 2-mm decrease in posterior condylar offset, the maximal obtainable flexion was reduced by a mean of 12.2° (⊡ Fig 29-7) Especially in TKA using anterior referencing for femoral component positioning, the surgeon may have the tendency to reduce femoral condylar offset, since most surgical technical guides suggest the use of a smaller femoral component when in between two sizes By downsizing, overfilling of the flexion space with prosthetic material is avoided,and flexion is believed to be facilitated by greater laxity Our observations not confirm this reasoning,however As well as risking an excessive flexion gap, downsizing with anterior referencing leads to a decreased posterior condylar offset and therefore to reduced flexion because of earlier impingement (⊡ Fig 29-8) Femoral Roll-back ⊡ Fig 29-5 Mechanical impingement of the tibial insert against the back of the femur, blocking further flexion Posterior insert impingement is theoretically avoided by femoral roll-back Today, however, there is abundant evidence that the majority of contemporary PCL-retaining TKAs not show roll-back, but instead demonstrate socalled paradoxical roll-forward of the femoral component during flexion [9, 10] In a recent study using three-dimensional computeraided design video-fluoroscopy,we measured the amount of femoral roll-back in 121 knees, treated with 16 different types of arthroplasties with clinically excellent results, ⊡ Table 29-2 Quantitative influence of surgical factors on maximal flexion Factor Tibial slope rior condyles, blocking further flexion 1.7° extra flexion for every degree tibial slope Posterior condylar offset ⊡ Fig 29-6 Protruding bone beyond the femoral component’s poste- Effect 6° extra flexion for every millimeter posterior condylar offset Femoral roll-back 1.4° extra flexion for every millimeter femoral roll-back 29 192 IV Surgical Technique 80 60 postop minus preop flexion 40 y = 6,1553x + 14,592 R2 = 0,5834 20 -20 -40 -60 -12 -10 -8 -6 -4 -2 postop minus preop posterior condylar offset ⊡ Fig 29-7 Correlation of restoration of posterior condylar offset (postoperative minus preoperative) with postoperative flexion gain (+)/loss (-) Overlapping points are not shown (Reprinted, with permission, from [10]) 29 ⊡ Fig 29-8 Decreased posterior condylar offset (left) leads to earlier impingement and limited flexion (x@x’) (Reprinted, with permission, from [10]) during maximal knee flexion in a lunge activity.A highly significant correlation was noted between the amount of femoral roll-back and maximum weight-bearing knee flexion, with an average of 1.4° greater flexion for each millimeter of additional femoral roll-back (⊡ Table 29-2) Anterior Clearance While the posterior area of the knee is important with respect to avoiding early impingement and thereby maximizing postoperative flexion, the importance of the anterior aspect of the knee should not be underestimated Overstuffing the anterior compartment may lead to excessive tension on the anterior structures during flexion, with the risk of limited postoperative range of motion Increasing the patellar thickness for example, will lead to increased tension in the extensor mechanism,with anterior knee pain during flexion due to painful stretching of the quadriceps and patellar tendon, and limitation of flexion as a result The same negative effect is obtained when the femoral component is positioned too anterior, or when the anterior aspect of the component is thicker than the amount of bone that was resected Tibial component anteriorization should equally be avoided, since it will lead to patellar tendon and fat pad impingement against the frontal part of the insert during flexion, causing anterior knee pain and restricted range of motion Finally, an adequate release of the potentially shortened lateral retinacular expansion should be performed in order to facilitate maximal flexion Especially in the valgus knee with lateral tracking of the patella, such shortening needs to be addressed Conclusion Achieving maximal flexion after TKA is a continuous challenge for both the patient and the surgeon Despite the fact that the parameters influencing the achievement of maximal flexion have not been very well understood for a long period of time, recent work by our group and others has substantiated the importance of several surgical and design-related factors Many of these factors can indeed be controlled by the arthroplasty surgeon and are appropriately addressed during the surgical procedure 193 Chapter 29 · Achieving Maximal Flexion – J Bellemans When faced with a patient showing poor flexion following TKA, the surgeon ought to defer blaming the patient or his physiotherapist or the implant designer until he is convinced that the above-mentioned surgical technical factors have been addressed appropriately References Anouchi Y et al (1996) Range of motion in total knee replacement Clin Orthop Rel Res 331:87-92 Kim J et al (1994) Squatting following total knee arthroplasty Clin Orthop Rel Res 313:177-186 Lizaur A et al(1997) Preoperative factors influencing the range of movement after total knee arthroplasty for severe osteoarthritis J Bone Joint Surg [Br] 97:626-629 Jordan L et al (1995) Early flexion routine An alternative method of continuous passive motion Clin Orthop Rel Res 315:231-233 Schurman D et al (1985) Total condylar knee replacement A study of factors influencing range of motion as late as two years after arthroplasty J Bone Joint Surg [Am] 67:1006-1014 Akagi M et al (1997) Improved range of flexion after total knee arthroplasty Bull Hospital Joint Dis 56:225-232 Akagi M et al (2000) The bisurface total knee replacement: a unique design for flexion J Bone Joint Surg [Am] 82:1626-1633 Banks S et al (2003) Knee motions during maximum flexion in fixed and mobile bearing arthroplasties Clin Orthop Rel Res 410:131-138 Banks et al (2003) Making sense of knee arthroplasty kinematics: news you can use J Bone Joint Surg [Am] 85:64-72 10 Bellemans J et al (2002) Fluoroscopic analysis of the kinematics of deep flexion in total knee arthroplasty The influence of posterior condylar offset J Bone Joint Surg [Br] 84:50-53 11 Robyns et al (2003) Effect of tibial slope on flexion in total knee arthroplasty Proc Am Acad Orthop Surg 4:258 12 Bertin K et al (2002) In vivo determination of posterior femoral rollback for subjects having a NexGen posterior cruciate-retaining total knee arthroplasty J Arthroplasty 17:1040-1048 13 Dennis D et al (1998) In vivo anteroposterior femorotibial translation of total knee arthroplasty: a multicenter analysis Clin Orthop Rel Res 356: 47-57 14 Stiehl J et al (2000) In vivo kinematic comparison of posterior cruciate ligament retention or sacrifice with a mobile bearing total knee arthroplasty Am J Knee Surg 13:13-18 15 Stiehl J et al (1990) Detrimental kinematics of a flat on flat total condylar knee arthroplasty Clin Orthop Rel Res 365:139-148 16 Uvehammer J et al (2000) In vivo kinematics of total knee arthroplasty: flat compared with concave tibial joint surface J Orthop Res 18:856-864 17 Walker P, Garg A (1989) Range of motion in total knee arthroplasty A computer analysis Clin Orthop Rel Res 262:227-235 18 Whiteside L (2004) Ligament balancing in total knee arthroplasty Springer-Verlag Berlin Heidelberg New York Tokyo 19 Bonnin M (1990) La subluxation tibial antérieure en appui monopodal dans les ruptures du ligaments croisés antérieure Etude clinique et bioméchanique Thèse Med., Lyon, n° 180 29 30 30 Assess and Achieve Maximal Extension R S Laskin, B Beksac Summary Inability to obtain full extension following knee arthroplasty is due to a combination of many factors Some factors are not under the control of the surgeon, and are related to patient morphology and disease Others are related to prosthesis design Many, however, are directly related to the surgical technique and are therefore controllable by the surgeon By attention to detail, the surgeon can have a direct effect on these factors and can maximize extension Introduction The major goal of total knee arthroplasty is relief of pain Almost as important, however, is the restoration of function,and that function depends primarily on an adequate arc of motion in the knee Extension and flexion following a knee arthroplasty are dependent upon a multitude of factors related to surgical technique, the implant used, the physical therapy program, and the patient him- or herself This chapter will discuss these factors and describe methods that the authors have used to maximize motion in extension.A subsequent chapter will deal with the subject of obtaining full flexion Why Do We Need Full Extension? During normal gait, the knee is at full extension at the time of heel strike and then gradually flexes during stance phase and swing phase [1] A patient whose knee cannot come into full extension must contract his quadriceps to prevent the knee from buckling during early stance, and this increases the work of walking.Whereas most patients after knee replacement have sufficient quadriceps strength to compensate in this manner when they first begin walking,with continued walking quadriceps fatigue can result in a limp and anterior thigh pain [2].When the knee does not come to full extension, the limb is functionally short This can cause a limp as well as pain in the back and in the ipsilateral hip and ankle For all these reasons, there- fore,the goal for the total knee surgeon is to obtain full extension in the reconstructed knee How Do We Determine if the Knee Is Fully Extended? At the onset,the authors would like to distinguish between two terms Extensor lag refers to an inability to actively extend the knee to the point where it can be passively extended (it is the difference between passive and active extension).A flexion contracture, on the other hand, is an inability to bring the leg to full extension passively Although regaining muscle power is important after knee replacement, very few patients have an extensor lag following primary surgery It is a flexion contracture that we are most concerned with during knee replacement To examine for full extension,the patient should be recumbent with both legs exposed and the heel on the table If the knee is fully extended, the examiner should not be able to pass any of his hand behind the knee in the popliteal space.The greater the flexion contracture,the more fingers the examiner should be able to pass under the knee Full passive extension of the knee can appear limited if there is hamstring spasm or tightness (for instance, in the patient with discogenic disease),especially if the knee is tested with the hip flexed It is for this reason that assessing extension while the patient is sitting with his leg hanging off the side of the examining couch often results in a false increase in the appearance of a flexion contracture If an exact measurement of extension is required, a lateral cross-table radiograph can be taken with the ankle supported on a small box The standard method of recording knee range of motion assigns zero degrees to the fully extended knee A knee that has a 5° flexion contracture and can, for example, flex to 125° of flexion should be listed as having a range of motion of 5°-125° The use of minus numbers should be reserved for degrees of recurvatum at the knee During surgery,these tests are difficult to perform because the leg is encased in sterile drapes A test has been used by the senior author that eliminates this problem: The leg is lifted from the ankle and the ankle joint itself is 195 Chapter 30 · Assess and Achieve Maximal Extension – R S Laskin, B Beksac passively dorsiflexed.Axial pressure is then applied to the sole of the foot If there is a flexion contracture, the knee will suddenly flex.If the knee is at full extension,however, there will be motion Although the presence of pain can lead to a false evaluation of joint motion, this relates predominantly to flexion In a study performed at the senior author’s institution,patients who were to undergo knee arthroplasty had an evaluation made of knee motion prior to and after the administration of their epidural anesthetic Although an average of 15° more motion was obtained in flexion once the patient’s pain sensation had been eliminated, there was no significant change in extension What Factors in the Arthritic Patient’s Knee Can Cause a Block to Full Extension? Lack of full extension is commonly seen in patients with advanced arthritis who are candidates for knee arthroplasty In the author’s database of over 1500 patients undergoing TKA,the average block to full extension in patients with osteoarthritis was 5°.In patients with rheumatoid arthritis the mean flexion contracture was 10.5°, while in patients with post-traumatic arthritis it was 14° It is fairly intuitive that this pre-operative contracture must be corrected at surgery if a postoperative contracture is to be avoided For patients with a flexion contracture less than 10°15°, the culprit is usually anterior or posterior osteophytes (⊡ Fig 30-1) Anterior tibial osteophytes are normally removed when the proximal tibia is resected; however, posterior femoral osteophytes, which can tent the ⊡ Fig 30-1 Multiple osteophytes ⊡ Fig 30-2 Removing posterior osteophytes posterior capsule, are not easily visible during the surgical exposure [3] Posterior femoral osteophytes can be most easily removed once the proximal tibia and posterior femur are resected A laminar spreader is placed medially, and the knee, in 90° of flexion, is distracted A curved osteotome and angled curettes will remove the posterior osteophytes from the medial femoral condyle (⊡ Fig 30-2) The position of the laminar spread is then changed to the medial side and a similar procedure is performed to remove any lateral femoral condylar osteophytes Finally, a check should be made for any remaining osteophytes behind the posterior cruciate ligament For patients with a flexion contracture >15°, further releases are normally necessary The next step should be elevation of the posterior capsule from the femur The knee should be flexed maximally and laminar spreaders again placed between the femur and tibia The posterior capsule of the knee can then be elevated for 1-2 cm from the proximal femur using a periosteal elevator [3] For flexion contractures greater than 45° this same approach can be used to elevate the tendinous origins of the gastrocnemius muscle medial and laterally In 1991, the senior author reported his results using a technique of transverse sectioning of the posterior capsule [4], a technique that had initially been described by Insall [5] The safety of this procedure was based on the assumption that, in flexion, the posterior neurovascular structures displaced posteriorly away from the posterior capsule In actuality, the reverse is true, as described by Zaidi [6] in 1995 With knee flexion, the neurovascular bundle is displaced anteriorly and can lie tethered against the posterior capsule For this reason, posterior capsule sectioning should not be routinely used, lest inadvertent popliteal artery and vein damage occur An apparently simple surgical solution to correct a block to full extension would be to remove extra bone from the distal femur, i.e., a segment of bone greater than the distal thickness of the femoral component that will be inserted Whereas an extra resection of 3-4 mm can at 30 196 IV Surgical Technique times be beneficial to help correct a block to full extension, further resection than this should usually not be performed.Doing so raises the joint line and adversely affects knee kinematics It can also result in an extensor lag and, in the extreme, damage to the collateral ligament insertions on the femur What Factors in the Implant Itself May Lead to a Block to Full Extension? All currently available total knee components, if they are properly aligned and positioned in the knee, allow complete knee extension However, the surgeon has to be knowledgeable of the configuration of the tibial component in choosing the degree of posterior tibial slope that is to be created.For example,if the anterior portion of the tibial component is “built up”,as is the case with some ultracongruent inserts, a posterior slope of the resection will lead to anterior impingement and a block to full extension In these cases full extension is normally obtained by resecting the tibia at 90° to its anatomical axis in the sagittal plane,rather than the usual 3°-5° backslope 30 What Factors of Surgical Technique May Lead to a Block of Full Extension? Problems with the surgical technique itself can result in a block of full extension even in a patient who had full extension prior to the knee replacement This occurs because of stuffing of the extension space The borders of the extension space are the resected surface of the distal femur and proximal tibia The thickness of the space is related to the amount of bone and cartilage that has been removed and the elasticity of the surrounding capsular structures It is this space which must be filled with implants of proper thickness if the knee is to be stable in extension.Likewise,overfilling of this space can lead to a potential block of full extension In most situations, the surgeon will remove bone equal in thickness to the distal thickness of the femoral component to be inserted Doing this positions the prosthetic joint line at its proper proximal-distal level and enhances knee kinematics and patellar tracking [7] Since the distal thickness of different implants vary (normally between and 12 mm), the amount of resection will vary dependent upon the implant itself The thickness of proximal tibia that is removed varies Most implant systems include some type of stylus device that senses the highest point on the “normal” tibial plateau and then positions the tibial cutting block a certain distance (normally 8-10 mm) below this This method is applicable to many situations in which there is a “normal” (or at least ⊡ Fig 30-3 Sensor, posterolaterally a less abnormal) side remaining on the tibial surface,i.e.,the lateral side in a patient with a varus deformity If one uses this method, the stylus must be placed at the lowest point of the normal side of the joint to judge the proper resection plane For a varus knee, the stylus is placed laterally Anatomically, the lateral tibial plateau is convex from front to back , and the lowest point is posterior, not in the center of the plateau (⊡ Fig 30-3) Placing the stylus in the center of the plateau will result in insufficient bone being removed and will lead to a stuffed extension space For a valgus knee, reference is made from the medial tibial plateau Anatomically, that plateau is concave anterior to posterior so that the stylus there can be placed in the center of the surface The method of using the stylus on the lowest point of the good side becomes ineffectual in the patient with inflammatory arthritis, where both the medial and lateral sides are often affected to the same degree In order to avoid this problem, the senior author has adapted a method [8] which is a combination of the measured resection and extension space filling methods The distal femur is resected as described above The knee is then extended and tensed medially and laterally with laminar spreaders.A spacer block,equal to the sum of the distal dimension of the femoral component and the thinnest tibial component,is then set at the level of the cut surface of the femur The inferior surface of the block marks the level for the tibial resection A block to full extension can also occur if the components are malpositioned.Although small degrees of malposition usually cause no statistical difference in the arc of motion [9], larger amounts can result in a flexion contracture For example, the senior author has seen cases where flexion of the femoral component greater than 15°-20° from the anatomical axis of the femur in the sagittal plane rendered the knee unable to extend fully A situation often occurs where the knee fully extends with the trial implants in place and a thigh tourniquet in- 197 Chapter 30 · Assess and Achieve Maximal Extension – R S Laskin, B Beksac flated However, after the tourniquet is released and the incision is closed, there appears to be a flexion contracture This pseudo flexion contracture is secondary to a hemarthrosis.As such, in most cases it abates as the postoperative hemarthrosis abates Will a flexion contracture that remains at the end of the operation prior to tourniquet release gradually stretch out with time? Although there have been reports of this occurring [10, 11] this has not been the authors’ experience The degree of extension that is present with the implants in place and with the tourniquet inflated is most often the maximum extension that the patient will finally obtain On occasion, one encounters a patient who has undergone a revision operation during which his surgeon has “changed the polyethylene”, inserting a thinner component in order to preclude a flexion contracture One must seriously question whether full extension was ever obtained at the original surgery The soft tissues of the knee are viscoelastic If one presses hard enough with the patient under anesthesia it may appear that the knee is extending; however, as soon as the pressure is removed the knee will “spring back” into a mild flexion contracture During testing of the knee, it should be allowed to come to full extension without pressure being placed on the patella lest a false evaluation be obtained Although full extension is the goal, there are situations in which full extension is not possible without marked shortening of the femur Such might be the expected situation if knee arthroplasty were performed in patients with flexion contractures >60° [12] Such situations were occasionally encountered in the early years of TKA, when patients would present to the surgeon after having been chair bound and nonambulatory for many years With the knowledge of joint replacement that now exists among the medical and lay communities, seeing a patient at that late stage has become uncommon.For such a patient, pre-operative traction can often decrease the contracture to below 45° The senior author has elected in those uncommon situations either to perform a femoral shortening and to accept an extensor lag or to allow a residual 10°-15° flexion contracture to remain rather than risking stretch injuries of the neurovascular structures The use of Botox injections into the hamstring muscles has recently been suggested for patients with severe flexion contractures secondary to cerebral palsy This treatment may offer some promise for the patient with a severe pre-operative contracture in whom full extension is not possible at surgery without excessive femoral shortening Postoperative Factors Despite proper surgery, a block to full extension may occur following surgery.If,for example,the patient uses pillows or other bolsters under the knee on a repeated basis during the first few postoperative weeks, a flexion contracture can develop A flexion contracture can also develop if the patient is allowed to sleep in a continuous passive motion (CPM) machine Observations of patients sleeping in a CPM unit will often reveal that the knee joint moves from the axis of flexion of the machine and never comes to full extension This problem has become less prevalent now that CPM machines are not routinely used 23 h a day, a method that was recommended during the 1980s Problems in Adjacent Joints There are patients with bilateral knee osteoarthritis and severe angular or flexion deformities who, because of concomitant medical problems,undergo the knee arthroplasty during two separate hospital stays, rather than simultaneously The knee that has been operated upon first becomes longer than the contralateral side To compensate for the leg length discrepancy, the patient will walk with the operated knee slightly flexed,and over a period of several months this can lead to a flexion contracture.The treatment is to place a lift on the shoe of the nonoperated leg until the time of its surgery A similar problem can occur if the patient has a severe hip flexion contracture secondary to coxarthrosis It most cases it is beneficial to treat the hip first to allow full extension and then, at a later time, perform the knee arthroplasty References Perry J (1990) Pathologic gait Instr Course Lect 39:325-331 Tew M, Forster IW (1987) Effect of knee replacement on flexion deformity J Bone Joint Surg [Br] 69:395-399 Lombardi AV (2001) An algorithm for PCL in TKA Clin Orthop Rel Res 392: 75-87 Laskin RS The PS total knee prosthesis in the knee with severe fixed varus deformity Insall JN, Scott WN, Ranawat CS (1979) The total condylar knee prosthesis A report of two hundred and twenty cases J Bone Joint Surg [Am] 61:173-182 Zaidi SH, Cobb AJ, Bentley G (1995) Danger to the popliteal artery in high tibial osteotomy J Bone Joint Surg [Br] 77:384-386 Yoshii I, Whiteside LA, White SE, Milliano MT (1991) Influence of prosthetic joint line position on knee kinematics and patellar position J Arthroplasty 6:169-177 Laskin RS (1991) Soft tissue techniques in total knee replacement In: Laskin RS (ed) Total knee replacement Springer-Verlag, London, pp 4153 Ritter MA, Stringer EA (1979) Predictive range of motion after total knee replacement Clin Orthop 143:115-119 10 Tanzer M, Miller J (1989) The natural history of flexion contracture in total knee arthroplasty A prospective study Clin Orthop 248:129-134 11 Mc Pherson EJ, Cushner FD, Schiff CF, et al (1994) Natural history of uncorrected flexion contractures following total knee arthroplasty J Arthroplasty 9:499-502 12 Lu H, Mow CS, Lin J (1999) Total knee arthroplasty in the presence of severe flexion contracture: a report of 37 cases J Arthroplasty 14:775-780 30 ... every millimeter femoral roll-back 29 192 IV Surgical Technique 80 60 postop minus preop flexion 40 y = 6, 155 3x + 14 ,59 2 R2 = 0 ,58 34 20 -2 0 -4 0 -6 0 -1 2 -1 0 -8 -6 -4 -2 postop minus preop posterior... alignment guides on final total knee arthroplasty component position J Arthroplasty 13 :55 2 -5 58 Moreland JR (1988) Mechanisms of failure in total knee arthroplasty Clin Orthop 226:4 9-6 4 Lotke P et al... Anterior translation in mm (SD, range) 6.3 (0.6, 5. 5- 1 0.0) 0.6 (0.6, 0. 0-2 .5) 7.6 (1.1, 5. 0-1 1.0) 2.2 (1.7, 0. 0-8 .5) 9 .5 (1.8, 6. 5- 1 4.0) 4.3 (2.0, 0. 0-1 0.0) Flexion gap size expressed in mm polyethylene;